In response to recent environmental protection movement, a regulation of exhaust gas such as carbon dioxide gas which is a cause of global warming has been strengthened. This is the reason why electric vehicles (EVs) and hybrid electric vehicles (HEVs or PHEVs) are actively developed in place of automobiles using fossil fuels such as gasoline, diesel fuel oil, and natural gas in the automotive industry. Nickel-hydrogen secondary batteries and lithium ion secondary batteries are used as these EV, HEV, or PHEV secondary batteries. In recent years, many nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries have been used because light-weight and high-capacity batteries can be obtained.
Generally, as EV, HEV, or PHEV secondary batteries, many prismatic secondary batteries produced by accommodating, in a prismatic outer can, a flat electrode assembly obtained by laminating or winding a positive electrode plate and negative electrode plate with a separator interposed therebetween are used. Since a large-current and high-output discharge operation is performed during the sudden acceleration or uphill travel of a vehicle, it is desired to reduce the internal resistance of a battery to the utmost. Though there are a mechanical caulking method, ultrasonic welding method, laser welding method, and resistance welding method as the method for collecting current by electrically bonding the electrode substrate exposed portion of an electrode plate with the current collector, a resistance welding method is suitable for batteries which need a large current and high output because this method enables economical production of a battery and production of a battery having a low resistance, and production a battery resistant to variations with time.
On the other hand, the number of positive electrode substrate exposed portions and negative electrode substrate exposed portions to be laminated in EV, HEV, or PHEV prismatic secondary batteries becomes very large because of a large capacity. Further, in EV, HEV, or PHEV nonaqueous electrolyte secondary batteries, aluminum or an aluminum alloy is used as many positive electrode substrates and positive electrode collectors and copper or a copper alloy is used as many negative electrode substrates and negative electrode collectors. These aluminum, aluminum alloys, copper, and copper alloys are materials having a small electric resistance and high heat conductivity and therefore need large welding energy to carry out resistance welding between the positive electrode substrate exposed portion and the positive electrode collector and between the negative electrode substrate exposed portion and the negative electrode collector without fail, to increase welding strength, thereby reducing the internal resistance of the welded part.
Sputters occur at the contact part between the electrode rod and current collector when the electrode substrate exposed portion of the electrode plate is bonded with the current collector by resistance welding. In order to prevent these sputters from diffusing to the electrode assembly side, a lib for preventing sputter diffusion is disposed on the electrode assembly side of the current collector. For example, Patent Literature 1 shown below discloses a method for producing a prismatic secondary battery in which, as shown in FIG. 8, a pair of current collectors 52 and 53 is arranged on each side of an electrode substrate exposed portion 51 of an electrode plate of a laminated or wound electrode assembly 50, a pair of resistance welding electrode rods 54 and 55 is brought into contact with both sides of the pair of current collectors 52 and 53 to conduct a welding operation while pressing the pair of resistance welding electrode rods 54 and 55 against each other.
Ribs 52a and 53a extended in a direction perpendicular to the electrode assembly 50 are formed on the electrode assembly 50 sides of the pair of current collectors 52 and 53 respectively. These ribs 52a and 53a are so formed that sputters 56 generated in the resistance welding between the electrode rod 54 and the current collector 52 and between the electrode rod 55 and the current collector 53 are each prevented from reaching the electrode assembly 50 side to thereby shield the electrode assembly 50. Ring-shaped insulation tapes 57 and 58 disposed between the current collector 52 and the electrode substrate exposed portion 51 and between the electrode substrate exposed portion 51 and the current collector 53 respectively serve to collect the sputters generated in the resistance welding between the current collector 52 and the electrode substrate exposed portion 51 and between the electrode substrate exposed portion 51 and the current collector 53 respectively in these ring-shaped insulation tapes 57 and 58.